I added a second axle to my trailer, additionally I underslung the springs, this got me about 4" closer to the ground.
I use 205/75/R15 tires. This trailer is scary with a single axle on the highway!! I tow at 75 with my 5spd 5.0 F150 now and not a worry at all.
I think the conversion cost me about $4-500, plus two more tire/wheels. I don't regret it at all.
Also, tounge weight. The boat weighs over 3k with the trailer. Does your tounge weigh 300 lbs? Because mine (when i got the boat) was nowhere near, that helped a LOT as well. (There are 4 u-bolts you can loosen, jack the trailer up onto 4 jackstands, slide the axle back about 4-5 inches.